This invention relates to cathode ray oscilloscopes (CROs) in general and, in particular, to a sweep generator for generating a sawtooth sweep signal for application, typically as a time base, to a deflection system of a cathode ray tube (CRT) forming a part of the CRO. The invention pertains more specifically to such a sweep generator featuring provisions for eliminating the small, rapid variations in the CRT waveform display which are known to the specialists as jitter.
In a typical conventional sweep generator (shown in FIG. 1 of the drawings attached hereto), a sweep gate as in the form of a flip flop responds to the first incoming trigger pulse after a holdoff signal has acquired a predetermined state, as by going low. The sweep gate when so set causes a sawtooth generator to generate a sweep ramp, which is applied to the deflection plates (usually horizontal) of the CRT to give a beam displacement across the screen.
A problem with this type of sweep generator is that the sweep gating flip flop becomes unstable in operation a trigger pulse is supplied concurrently with the moment when the holdoff signal goes low. The unstability of the sweep gating flip flop manifests itself as the jitter of the display on the CRT screen.
One known expedient for eliminating the jitter has been to connect a tunnel diode between the output line of the holdoff circuit and ground for a faster switching speed. A change in value of the holdoff signal will concur less with the trigger pulses, so that jitter will become less pronounced. However, the tunnel diode is expensive, thermally unstable, and not fully reliable in operation.
Another conventional remedy for the problem has been the use of two sweep gating flip flops (FIG. 2). Both flip flops directly input the trigger pulses. The first flip flop further has a reset input connected to the holdoff circuit, whereas the second flip flop has a reset input connnected to the first flip flop and has an output connected to the sawtooth generator. The second flip flop causes the sawtooth generator to produce a sweep ramp in response to the second trigger pulse after the holdoff signal has gone low. Since the second flip flop is thus never triggered at the same time when the holdoff signal goes low, no jitter is to take place due to this reason.
However, the above second conventional solution has the following weaknesses:
1. Two flip flops are required that can both respond to the trigger pulses of a high recurrence rate.
2. The trigger circuit must feed the two flip flops with one and the same trigger signal.
3. In high frequency operation a setup time of approximately one nanosecond is required from the moment the second flip flop is released from the reset state to the moment it becomes capable of accepting trigger pulses. Another approximately one nanosecond is required from the moment the first flip flop is triggered to the moment its output changes accordingly. Thus the second flip flop tends to become unstable in operation at a frequency of approximately 500 megahertz, a reciprocal of the sum (about two nanoseconds) of the above two delay periods. This unstable operation of the second flip flop has frequently resulted in jitter in the neighborhood of that frequency.